Process of potting electrical apparatus



y 31, 1960 J. G. HOPPE 2,938,313

PROCESS OF POTTING ELECTRICAL APPARATUS Filed June 6, 1957 2 Sheets-Sheet 1 [unto/1' May 31, 1960 J. G. HOPPE 2,938,313

PROCESS OF POTTING ELECTRICAL APPARATUS 2 Sheets-Sheet 2 Filed June 6, 1957 [/7 Mw to)": c/6/7/7 6.2/90 9,

Ms Attor-mgy- United States Patent PROCESS OF PO'ITING ELECTRICAL APPARATUS John G. Hoppe, Fort Wayne, Ind., assignor to General Electric Company, a corporation of New York Filed June 6, 1957, Set. No. 664,123

2 Claims. (Cl. 53-22) This invention relates to the process of potting electrical apparatus with hardenable impregnating materials such as asphalt, and more particularly to a process for potting electrical apparatus of the type in which sand is used as a filler for the impregnating material in order to improve the heattransfer characteristics of the apparatus.

Certain electricalrapparatus, such as transformers, are housed in an enclosing case. In order to improve the heat transfer characteristics of the apparatus, it has been customary to fill the case with a hardenable insulating material, such as asphalt, referred to as potting and it has also been common separately to impregnate the core and coil assembly of the transformer with such material by means of an impregnating treatment. One important impregnating treatment whichhas been employed is that described in Patent 2,872,344of Robert D. Mees and assigned to the assignee of the present application.

In order further to improve the heat transfer characteristics of such apparatus, it has been common practice to load the asphalt with a granular material, such as sand prior to pouring it into the case. As a specific example, hermetically sealed transformers have been in the past treated and potted by initially impregnating the core andcoil assembly withasphaltin accordance with the process described in the aforementioned Mees patent, assemblying thev impregnated core and coil assembly in an enclosing case leaving. a small opening as the only entry into the case, pouring hot asphalt loaded with approximately percent by weight 300 mesh silica into the case through the opening therein until the case is filled, and finally securing a cover on the pouring opening completely to seal the case.

While the foregoing method has been generally satisfactory, it has long been considered desirable to increase the sand content of the asphalt above 50 percent by weight in order to further improve the conductivity and to improve the vibration resistance of the apparatus. It has further; been-found that with the present impregnation and potting process,-,many void spaces were present particularly in the area immediately surrounding the core and :coil' resulting from dr aining-oif of the impregnating material leavingspaces or pockets which were not filled by the subsequent potting operation; these void spaces-are particularlydetrimental where high corona resistance is required. 'In. addition, it has been considered desirable to combine the impregnating and potting processes into a single process. Prior efforts to i11 crease the sand content above 50 percent by'weight have been unsuccessful since the viscosity of the asphalt becomes too high to permit pouring. It isflthereforev an 'object' of, this invention to provide an improved process for potting-electrical apparatus.

Another object of this invent'ion is to provide an improved process for potting electrical apparatus with hardenable' i'rnp'regnating material having a granular material filling which provides a higher content of such granular material than priorprocesses;

A further object of this invention is to provide a process for simultaneously impregnating and potting electrical apparatus.

Further objects and advantages of this invention will become apparent by reference to the following description and the accompanying drawing, and the features of novelty which characterize this invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In order to provide a higher sand content in the potting material and also to combine the impregnating and potting operations, I proposed placing an unimpregnated core and coil assembly in the enclosing case, completely filling the case with sand, and then forcing the impregnating material into the case under apressure differential, for example in accordance with the disclosure of the aforementioned Mees application. I have found that with this approved process, a sand content of approximately percent by weight can be provided in contrast with the maximum of 50 percent by weight obtainable when the asphalt was preloaded with the sand. I have further surprisingly found, contrary to any expectation, that with this higher sand content, the asphalt will not soften and flow out of the case even though the case is punctured and raised to temperatures far in excess of the normal flowing temperature of the asphalt.

The invention together with additional objects and advantages will be best understood from the following description of a specific embodiment, when read in connection with the accompanying drawing in which:

Fig. 1 is an exploded view of a typical hermetically sealed transformer prior to assembly with the case;

Fig. 2 illustrates one step in the process according to one embodiment of the invention;

Fig. 3 illustrates another 'step of the process according to one embodiment of the invention;

Fig. 4 is an exploded view of one step according to one embodiment of the instant invention;

Fig. 5 illustrates a step in the process according to one embodiment of the instant invention;

Fig. 6 illustrates the impregnation step of the instant invention;

Fig. 7 illustrates another step of the invention according to one embodiment thereof; and

Fig. 8 illustrates another step of the invention according to a further embodiment thereof.

Referring now to the drawings, in Fig. 1 there is shown a typical hermetically sealed transformer having a core and coil assembly 1, secured to a cover 2 for the enclosing case 3, with a plurality of suitable external terminals 4 projecting through the cover 2 and being connected to the coils of the core and coil assembly by suitable leads (not shown). Enclosing case 3 has a recessed opening 5 formed therein.

In the first step of my improved process, as shown in Figs. 1 and 2, the cover 2 with the core and coil assembly positioned thereon is assembled in the open end 6 of the enclosing case 3 and the cover is secured to the enclosing case 3 in any suitable manner, as by soldering with induction heating by means of a suitable induction heating coil 7. The opening 5 remains open and as shown in Fig. 3, the assembly is now turned on end and the enclosingcase 3 completely filled with sand through the opening 5. In order to achieve substantial filling of the enclosing case 3 with sand, it may be found desirable to vibrate the assembly by means of a conventional vibrating table 8actuated by a driving motor 20. As shown in Figs. 4 and 5, the opening 5 is next closed by a plug 9' having a smaller opening 10 formed therein, plug 9 being seated in the recessed opening 5 and soldered in position.

As shown in Fig. 6, enclosing case 3 with the smaller opening 10 remaining open is then lowered by means of a suitable tray 11 into an impregnating tank 12. Impregnating tank 12 is provided with means for heating the same, such as, steam line 13, a hardenable impregnating material input line 14, a pressure applying line 15, a vacuum line 16 connected to suitable vacuum pumping apparatus 17, an impregnating material drain line 18 and a cover member 19 adapted to be sealed on the tank 12. Suitable valves are provided for opening and closing the various lines in the proper sequence as is well known in the art. After the case 3 on the tray 11 is positioned within the tank 12, the lid 19 is closed. Tank 12 is then filled with heardenable impregnating material, such as asphalt through line 14, and the material is forced into the case 3 through the smaller opening 10 to fill the case and impregnate the core and coil assembly 1 by means of a pressure differential established by a suitable combination of vacuum drawn through line 16 and pressure applied through line 15. After completion of the potting and impregnation process, the impregnating material is drained from tank 1 2 through line 18, the cover 19 is opened and the case 3 removed. The impregnating material is then hardened, in the case of asphalt merely by cooling, and the small opening 10 is closed, as by soldering as shown in Fig. 7. The excess impregnating material which may have adhered to outer surface of case 3 is then removed by any conventional degreasing process, for example as by spraying with trichloroethylene as shown in Fig. 8.

I have found that in the case of relatively small hermetically sealed transformers potted and impregnated in accordance with my improved process, that an opening 5 approximately inch diameter is adequate for admitting the sand. l-have found that 400 mesh silica produces a highly satisfactory product; when a finer mesh is used the device cannot be impregnated since the sand packs too tightly together and cannot be penetrated by the impregnant' and on the other hand, if a coarser mesh is used, the sand content cannot be made sufficiently high. I have further found that a inch diameter opening is adequate for introducing asphalt to the interior of the case.

I have found that it is essential that the asphalt be forced into the case and into the sand and core and coil assembly under a pressure differential; merely pouring hot asphalt into the sand will not even impregate the sand much less impregate the core and coil assembly. I have successfully used a pressure-vacuum impregnating process as shown in Fig, 4 of the aforementioned Mees application to impregnate and pot hermetic transformers. More specifically, the case 3 is placed in the tank 12, the lid 19 closed and sealed, and the tank with the transformer therein is then heated at approximately 150 degrees C. for 30 minutes. 20 inch vacuum is then pulled and released several times and the tank is then flooded with asphalt preheated to 150 degrees C. to above the top level of the transformer with existing vacuum. The vacuum is then pulled, followed by the application of nitrogen pressure at 80 pounds per square inch, the pulling vacuum and subsequent application of positive nitrogen pressure being repeated several times. The asphalt is then drained from the tank, the transformer removed and after cooling the small opening 10 is soldered shut and the excess asphalt removed as above described.

It will be understood that in my improved process, other impregnating processes may be utilized so long as the impregnating material is forced into the case under a pressure differential.

It will now be seen that my improved process allows impregnation of the core and coil assembly and potting of the assembly within the enclosing case to be accomplished in a single operation in contrast with the two separate operations previously required. As previously was pointed out, I have found that with this process the asphalt can be filled with approximately 80 percent of said by weight as opposed to the no more than 50 percent of sand by weight previously obtained. I have found that this higher percentage of sand improves the heat transfer characteristics of the transformer by approximately 12 percent. It has further been found that this higher sand content provides superior vibration performance and that corona discharge is reduced in high voltage units thereby making it possible to manufacture many additional transformer designs with this process which were impossible to build by the prior process since they could not be made corona-free. It has further been found possible to completely fill certain types of transformers, for example those having vertically mounted core and coil assemblies, which were impossible to fill by the prior process. Most importantly, as is pointed out above, with this high sand content and forcing of the impregnating material into the said and core and coil assembly under a pressure differential, it has been found that the asphalt will not soften and run out when the case is punctured even though the temperature is raised far above the normal flowing temperature of the asphalt. To illustrate this phenomenon, I cast two blocks of asphalt in cylindrical containers, one having the asphalt preloaded with 50 percent sand and the other having the mold filled with sand and the asphalt introduced therein by the above referred to pressure-vacuum process thereby providing substantially percent sand content. These two blocks were remover from their molds and heated to degrees C. for one and a half hours. At the end of this initial period, the preloaded asphalt with 50 percent sand had substantially flattened while the other block having the 80 percent sand content showed no change whatsoever in its original configuration. The blocks were then heated for one and a half hours at degrees C. after which time the 50 percent sand preloaded asphalt block had the configuration of a pancake, being completely semi-fluid, whereas the other block having 80 percent sand content again showed no change from its original configuration. The block formed in accordance with my process and having 80 percent sand content was then heated for one hour at 175 degrees C., i.e., substantially above the degrees C. at which the asphalt was forced into the mold, and after this period the block was semiplastic but none of the asphalt had run out; the block made in the conventional manner could not be heated at this temperature since the asphalt would completely run out of the sand.

This improved process and particularly the unexpected result provides superior transformers which will not leak asphalt even though they are operated at above normal temperatures and their cases are punctured. Further, it will now permit such transformers to be designed for operation at considerably higher temperatures.

While my improved process is described in connection with the treatment of a transformer, it will be readily apparent that it is equally useable with other electrical apparatus such as relays and motors.

It will now be seen that my improved process has four major and distinct advantages over the prior processes; it permits coil impregnation and potting to be performed in a single operation, it provides superior vibration performance, it provides improved heat-transfer performance, and most importantly and unexpectantly it provides a unit in which the impregnating material will not run out even though the case is punctured and raised to a temperature in excess of the normal flowing temperature of the material.

While I have shown and described a specific embodiment of this invention, further modifications and improvements will occur to those skilled in the art. I desire therefore in the appended claims to cover all modifications which do not depart from the spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The process of potting and impregnating a core and coil assembly for electrical apparatus comprising the steps of: positioning said assembly in an open-ended enclosing ease; securing a cover on said open end of said case, said case having a relatively small opening remaini-ng herein; substantially filling said case with relatively fine sand while vibrating said case; closing said case opening with a plug having a smaller opening therein; placing said case in an impregnating tank; subjecting said ease with said assembly therein to a pressure-vacuum impregnation treatment with hardenable impregnating material substantially to fill said case and to impregnate said assembly; removing said case from said tank; closing said smaller opening after removal of the case from said tank; and removing excess material from the exterior of said case.

2. The process of potting and impregnating a core and coil assembly for electrical apparatus comprising the steps of: positioning said assembly in an open-ended enclosing case; securing a cover on said open end of said case, said case having a relatively small opening remaining herein;

substantially filling said case with approximately 400 mesh sand while vibrating said ease; closing said case opening with a plug having a smaller opening therein; placing said ease in an impregnating tank; subjecting said case with said assembly therein to a pressure-vacuum impregnation treatment with hardenable impregnating material substantially to fill said case and to impregnate said assembly; removing said case from said tank; closing said smaller opening after removal of the case from the tank; and removing excess material from the exterior of said case.

References Cited in the file of this patent UNITED STATES PATENTS 1,827,571 Fiene Oct. 13, 1931 1,947,085 Hill Feb. 13, 1934 2,047,260 Franklin July 14, 1936 2,126,532 Cate Aug. 9, 1938 2,337,678 Nowell Dec. 28, 1943 2,478,754 Dubilier Aug. 9, 1949 2,578,024 Steinmayer Dec. 11, 1951 

